We performed genetic algorithm search for the atomic structure of the
long Lime unsolved Si-IX phase. We found two new structures with space
groups of P4(2)/m and P-4, respectively, which have lattice parameters
in excellent agreement with the experimental data. The phonon
calculations showed that the P4(2)/m structure exhibits a soft phonon
mode, while the P-4 structure is dynamically stable. Our calculation
also showed that the P-4 structure is a meta-stable structure in a
pressure range from 0 to 40 GPa, The Si-IX phase could be a mixed phase
consisting of the P4(2)/m and the P-4 structures. Published by Elsevier
Ltd.